1. Field of the Invention
The present invention relates to a circuit adapted for providing, from a rectifying bridge, on the one hand, a D.C. power supply intended for supplying a load and, on the other hand, one or several low voltage power supplies for supplying secondary circuits such as control circuits.
The present invention especially aims at the supply of low power motors, that is, of a power smaller than 1 kilowatt, for example, motors which can be found in many industrial or household applications such as washing machines, vacuum cleaners, fans, etc. In such applications, two constraints are imposed. The first one is that the circuit must be of low cost, the second one being that the circuit sends back a minimum amount of electric noise onto the mains.
2. Discussion of the Related Art
The general architecture of a conventional motor control circuit is illustrated in FIG. 1. This circuit includes a rectifying bridge 1 connected across the mains via a filtering circuit 2. Between the positive and negative output terminals of the rectifying bridge is connected a capacitor of high value C intended for storing and smoothing the D.C. voltage to be supplied to a load. To control a reciprocating motor M, three bridge arms, each including two switches S1-S2, S3-S4, S5-S6 are, for example, provided. The opening and the closing of these switches to supply the inductive windings of the three-phase motor are determined by a control circuit including, on the one hand, adapter circuits 3, 4, 5 coupled to the pairs of switches, and on the other hand, a microcontroller .mu.C intended for applying the control signals to the various adapter circuits. Adapter circuits of type L6386 and a microcontroller of type ST92141, both sold by STMicroelectronics, may, for example, be used.
Further, the circuit must include means for supplying power to the microcontroller, and possibly other elements of the circuit. For this purpose, a low voltage power supply circuit (LPS) 7 providing for example two D.C. voltages V1 and V2, of respective values 15 V and 5 V, is provided. Preferably, low voltage power supply circuit 7 is connected between the positive and negative terminals of the same rectifying bridge as that which supplies the main circuit. Further, a power factor (PF) improvement circuit 9 is generally provided across the rectifying bridge.
Conventional circuits seldom include all the elements described hereabove. In the more specific case of a circuit such as shown, it is difficult to design an association of a power factor improvement circuit and of a low voltage power supply circuit of low general cost. It should be recalled that the aim of power factor improvement circuit 9 essentially is to widen the current pulse which is extracted from the mains each time capacitor C is desired to be recharged, that is, upon each halfwave, to reduce the current harmonics sent back onto the mains, and thus to reduce the dimensions of the components (inductance and capacitance) of filter 2, which also helps to reduce costs. Indeed, presently, in practice, the sum of the costs of the low voltage power supply circuit and of the power factor reduction circuit assembly (if provided) and of the filter substantially amounts to half the cost of the entire circuit.